The present invention is directed to a method and apparatus for generating and placing foam cement across a long interval behind casing in a wellbore. The invention is more particularly directed to a method and apparatus for generating foam cement downhole in the casing.
Hydraulic cement slurries are commonly utilized in subterranean well operations. For example, hydraulic cement slurries are used in primary well cementing operations whereby strings of pipe such as casing and liners are cemented in wellbores. In performing primary cementing, a hydraulic cement slurry is pumped into the annular space between the walls of a wellbore and the exterior surfaces of a pipe string disposed therein. The cement slurry is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein. The cement sheath physically supports and positions the pipe string in the wellbore and bonds the exterior surfaces of the pipe string to the walls of the wellbore whereby the undesirable migration of fluids between zones or formations penetrated by the wellbore is prevented.
In well applications, the cement slurries must often be lightweight to prevent excessive hydrostatic pressure from being exerted on subterranean formations penetrated by the wellbore whereby the formations are unintentionally fractured. As a result, a variety of lightweight cement slurries, including foam cement slurries, have been developed and used. In addition to being lightweight, a foam cement slurry contains compressed gas which improves the ability of the slurry to maintain pressure and prevent the flow of formation fluids into and through the cement slurry during its transition time, i.e., the time during which the cement slurry changes from a true fluid to a hard set mass. Foam cement compositions are also advantageous because they have low fluid loss properties. Foam cement slurries often include various surfactants known as foaming agents to facilitate the foaming of the cement slurry when gas is mixed therewith. Other surfactants known as foam stabilizers may be added for preventing the foam slurries from prematurely separating into slurry and gas components. The foam slurry comprises a hydraulic cement, water present in an amount sufficient to form a pumpable slurry, a mixture of foaming and foam stabilizing surfactants to form and stabilize the foam cement slurry, and sufficient gas to foam the slurry. As is known in the art, a variety of other additives may be added to the slurry to provide desired characteristics.
The present means of foaming the slurry generally comprises mixing cement and a gas, preferably nitrogen, at low pressures and injecting the slurry into the well. The gas utilized to foam the cement slurry, which may be air or nitrogen, but is preferably nitrogen, is typically added to the cement at the surface, and the slurry is then pumped downhole through the casing and then into the annulus between the casing and the wellbore. Gas must be added in an amount sufficient so that the foam slurry, when it is in place between the casing and the annulus, has a quality of approximately 18% to 38%. In other words, nitrogen must be present in the range of up to approximately 38% by volume of the slurry at the desired pressure. If the quality of the cement varies from the desired range, the cement bond and strengths may be unacceptable.
For a fixed foam quality, the amount of nitrogen in the slurry will vary drastically from the top of cement (TOC) to the tail, or bottom of cement, due to hydrostatic pressure. Generally, at some point in the well, the pressure of the slurry is below 500 psi. If the cement is designed to have 35-38% quality at 500 psi, then quality at depths in the well where the pressure is greater than 500 psi will be lower. For example, if the well is about 10,000 feet deep, with a bottom hole pressure of about 5150 psi, the desired cement density may be about 10 lb/gal, which corresponds to about 37% quality. If at 500 psi the quality is 38% then Vgas/(Vgas+Vliquid)=0.38, or Vliq=1.63 Vgas. When pressure is increased to 5150 psi, gas volume changes to 500 Vgas/5150. The new quality is thus 0.097 Vgas/(0.097 Vgas+Vliq)=0.097 Vgas/(0.097 Vgas+1.63 Vgas)=0.056 or 5.6%. Thus, the desired quality is not achieved. If the cement is designed for 37% quality at 5150 psi, then Vliq 1.63 Vgas at 5150 psi. When pressure decreases from 5150 to 500, gas volume changes to 5150 Vgas/500=0.863 or 86.3%. Such a quality is unstable, and gas bubbles will break into larger gas bubbles. It is therefore difficult and sometimes impossible to achieve desired cement quality using present technology. Thus, there is a need for a method and apparatus that will provide consistent cement foam qualities from the top of the cement to the tail or trailing edge.
The current invention provides a method and apparatus for the generation of and placement of foam cement. The method includes displacing cement downwardly through a casing so that it exits the casing and enters an annulus between the casing and a wellbore. A gas is injected into the cement at a location downhole in the casing to foam the cement. Once a sufficient amount of cement has been displaced to fill the annulus between the casing and the wellbore, the flow of cement is stopped. The method may include placing nozzles downhole in the casing and injecting the gas through the nozzles. The placement step may comprise connecting a ported sub to a tubing and lowering the tubing into the casing until the ported sub is positioned at a desired location in the casing. The ported sub may have openings therethrough, which may be referred to as nozzles, through which the gas is injected. A tubing dead string may be connected to the ported sub, and the tubing lowered into the casing until it engages the bottom of the casing, which may comprise a float shoe. If the casing includes a float collar above a float shoe, the tubing will engage the float collar. Once the tubing engages the float apparatus, whether a float collar or float shoe, the tubing is then lifted to provide clearance between the end of the tubing and the float apparatus. The injecting step will thus comprise injecting nitrogen through the tubing and through openings in the ported sub into the annulus between the casing and the tubing to foam the cement.
A bottom cementing plug is preferably placed in the casing ahead of a leading edge of the cement. The bottom cementing plug may have outer wipers and inner wipers so that the bottom cementing plug will wipe the inner surface of the casing and the outer surface of the tubing utilized to lower the ported sub into the casing. The injecting step preferably begins when the bottom cementing plug passes the ported sub. The rate at which nitrogen is injected into the cement may be increased from the leading edge to the trailing edge of the cement so as to acquire a consistent cement quality once the cement is placed in the annulus between the casing and the wellbore. Preferably, the rate of nitrogen injected in the cement is increased at a constant rate from the leading edge to the trailing edge. A top cementing plug is placed in the casing behind the trailing edge of the cement and is displaced downwardly with a displacement fluid. Once the top cementing plug passes the ported sub, the injection of gas ceases.